The invention concerns new compositions of isocyanates containing diisocyanates having a diphenylethane structure and obtained from diphenylethane by the dinitration of aromatic rings, followed by a hydrogenation and a phosgenation. The compositions possess a lowered melting point as compared to the diisocyanates of dibenzyl already known in the literature; in addition they have the property of remaining fluid for several hours, which facilitates their use, especially in the production of polyurethanes. The distribution by weight of the diisocyanates in this composition amounts to 35 to 55% of the 2,4' isomer, 15 to 40% of the 4,4' isomer, 10 to 25% of the 2,2' isomer, 3 to 10% of the 3,4' isomer, and 3 to 8% of the 2,3' isomer.
Diisocyanates of diphenylmethane aromatic hydrocarbon structure are synthesized industrially (M.D.I.). They are obtained by the condensation of two molecules of aniline on one molecule of formaldehyde in the presence of an acid catalyst, followed by a phosgenation, leading to mixtures containing the 4,4', 2,4', and 2,2' isomers. But this condensation is never totally selective in 2-ring products and customarily mixtures of polyisocyanates and diisocyanates are synthesized, with the latter being separated by distillation in order to yield pure M.D.I. The 4,4' M.D.I. isomer is generally clearly predominant and possesses a melting point of 38.degree. C. Thus, depending on its content, it can be necessary to liquefy the diisocyanate mixture before being able to use it, either by melting it and working in the hot state or by converting it chemically.
Isocyanates of diphenylethane structure have already been described in the literature; we are dealing with diphenylethane-4,4'-diisocyanates whose preparation by phosgenation of the corresponding diamine has been published by E. COCEA, A. CARACULACU, C. MARCULESCU, A. PETRUS, I. MATEI in Studii si Cercetari Stiintifice-Chimie. Academia R. P. Romane Filiala Iasi (1959), 10, no. 2, p. 261. The phosgenation has likewise been described in Japanese Pat. No. 41-6583. After distillation, there is thus obtained a diisocyanate having a melting point of 88.degree.-89.degree. C., which necessitates working at an elevated temperature in order to manufacture polyurethanes.
Some publications treat applications of this diisocyanate. I. MATEI, E. COCEA, A. CARACULACU, A. PETRUS in Studii si Cercetari Stiintrifice-Chimie. Academia R. P. Romane Filiala Iasi (1960), 379 and D. J. LYMAN, J. HELLER, M. BARLOW in Die Makromolekulare Chemie 84, p. 64 (1965) have described the synthesis and certain properties of polyurethanes derived from diphenylethane-4,4'-diisocyanate.
The diphenylethane-2,2'-diisocyanate has also been cited in French Pat. No. 1,492,642, but its melting point of 78.degree.-79.degree. C. is likewise elevated. In can be prepared by oxidizing duplication of orthonitrotoluene, followed by a hydrogenation and then a phosgenation; however, the yield from the first stage is mediocre.
If the oxidizing duplication is carried out starting from a mixture of the isomers of nitrotoluene, then a mixture of dinitro isomers is obtained which it is possible to hydrogenate and phosgenate. But there again, the yield from the first stage is mediocre and it is the 4,4' isomer which is favored. There is thus obtained a mixture of diisocyanates in which this isomer predominates and which does not present the fluid properties of the compositions in conformity with the present invention.